This invention relates generally to lithium anode assemblies and to electrochemical batteries or cells incorporating such anode assemblies.
Lithium anode assemblies are used in electrochemical batteries in combination with various cathode materials. For the purposes of this invention, the most preferred cathode materials are comprised of charge-transfer-complexes which are the reaction product of an organic component and a halogen component. For example, such a cathode material may comprise the reaction product of a polyvinylpyridine polymer and a halogen. Specifically, the reaction product of poly-2-vinylpyridine and iodine are most preferred. 4-vinylpyridine polymer may also be used in place of or in addition to 2-vinylpyridine polymer. Other cathode materials such as those described in U.S. Pat. No. 3,352,720 to Wilson et al, U.S. Pat. No. 3,660,163 to Moser, U.S. Pat. No. 3,660,164 to Hermann et al and U.S. Pat. No. 3,674,562 to Schneider et al may also be used.
Although the most preferred halogen component is iodine, bromine and interhalogens such as iodine monobromide and the like may also be used for the cathode.
It should be understood that the various initial component materials referred to herein with respect to the cathode are described with reference to their identity as initially introduced into the pertinent preparation step. A wide variety of chemical interreactions may occur between various initial components of the cathode, depending on the preparation used, which render the resultant reaction products difficult to describe.
In electrochemical batteries of the type described herein, the electrochemical reaction occurs between the lithium anode and the halogen component of the cathode material. For example, if the halogen component is iodine, a solid electrolyte of lithium iodide will form in situ from the interaction of the halogen component of the cathode material with the metallic lithium anode. Electrochemical batteries of this type have found use as in power sources for implantable cardiac pacemakers.
In fabricating these batteries, some manufacturers contact the lithium anode, at least on the operative surface or surfaces thereof, with an organic material of the type utilized in the cathode. The prevalent contact arrangement is by way of coating the lithium anode at least on its operative surface or surfaces with the organic material in one or more layers. In cells having such coatings, preferably but not necessarily, the same organic material as is used in the cathode of the cell is used for the coating (in the sense that the initial materials may be the same).
A typical procedure for coating the lithium anode involves the painting thereon of a solution of the donor material. For example, poly-2-vinylpyridine dissolved in a suitable solvent such as benzene may be painted onto the anode. The subsequent evaporation of the solvent leaves the polymer remaining on the anode in the form of a coating. Multiple layers may be painted onto the anode to provide any desired coating thickness. U.S. Pat. Nos. 3,957,533; 4,071,662 and 4,117,212 to Mead et al describe such coated lithium anodes. The coating described therein is of an organic material preferably of a polyvinylpyridine polymer and in particular 2-vinylpyridine polymer. Additionally, 4-vinylpyridine and 3-ethyl 2-vinylpyridine polymers are described for the coating. The subject matter of these patents is incorporated herein by reference.
The coating of organic material is believed to result in a greater utilization of the surface of the lithium anode element by the cathode material of the battery and a reduction of cell impedance.
Another means of contacting the lithium anode operative surface with an organic material is described in U.S. Pat. No. 4,182,798 to Skarstad. In accordance with that patent, poly-2-vinylpyridine in self-supporting sheet-like form is positioned against the operative anode surface or surfaces and caused to adhere thereto by means of adhesive, pressure or the like. The subject matter of this patent is also incorporated herein by reference.
In fabricating an electrochemical cell or battery utilizing a coated or otherwise "contacted" lithium anode, the anode is placed in a suitable container with other cell components. The container is of a material which is non-reactive with the cell components and is molded or otherwise formed to a desired configuration. the cell components comprise mainly the lithium anode suitably positioned within the container and a cathode material of the type described hereinabove, preferably the reaction product of polyvinylpyridine and a halogen, the halogen most preferably being iodine. The cathode material operatively contacts the anode through the anode coating. The exact mechanism by which the halogen-containing cathode material and lithium come into operative contact through the coating is not known. However, the halogen, for example, iodine, of the cathode material reacts with the lithium of the anode to form the solid lithium iodide electrolyte in situ between the anode and cathode components.
It is desirable from the standpoint of battery capacity that the cathode material contain relatively large effective amounts of the halogen component relative to the organic component. For example, in the case of lithium iodine batteries utilizing cathode materials comprised of the reaction product of poly-2-vinylpyridine and iodine, a 20:1 initial ratio of iodine to poly-2-vinylpyridine as initially combined to prepare the cathode material has become more or less a standard for commercial production. It is desirable that the initial ratio be increased to 30:1, 50:1 and even 100:1 or higher where possible. The major advantage expected from such a change is a significant increase in deliverable capacity over 20:1 cathode units. However, in actual practice, the significant increase in deliverable capacity with higher weight ratios of halogen, iodine in particular, have not been fully achieved. One reason for this is that the higher weight ratio batteries have been found to be more susceptible to self-discharge than the lower weight ratio batteries. Self-discharge directly affects battery capacity detrimentally. Higher initial electrical impedance is also a problem with higher weight ratio batteries.
In most instances, the cells described herein will be assembled and encapsulated in a dry atmosphere, suitably in dry rooms or enclosures having a relative humidity less than about two percent, using substantially anhydrous and/or dried components. All of the components, cells and tests of cells described herein were prepared and performed substantially in such dry rooms. In production embodiments, the cells of the invention will preferably be enclosed in hermetically sealed enclosures such as welded stainless steel containers with appropriate electrical feedthrough arrangements for electrically contacting the cell components, as is known in the art. The assembly and encapsulation of such cells preferably is accomplished in a dry room using substantially anhydrous and/or dried components.